Optical filters are nearly ubiquitous elements in the field of optical communications. Optical filters are used for both multiplexing and demultiplexing, routing, attenuating, encoding, and decoding optical signals. Since optical signals are generally carried over long distance by optical fibres, optical filters are coupled in a well-known manner with the optical fibres, such that a filter assembly will be disposed in-line and between at least a pair of optical fibres. However, in some instances, these filters are configured such that they are essentially one-sided devices. For example, two wavelengths may be launched into a one sided device having two optical fibres at a same end; in this instance one wavelength may be extinguished wherein another wavelength will passed from one optical fibre to the other. Alternatively, a single sided optical filter may serve to attenuate light passing therethrough. In order to efficiently couple light from an optical fibre to a filter and back to an optical fibre, a lens is required; otherwise the light exiting an end face of an optical fibre will diverge and much of the light will be lost. It is generally desired to provide a collimated beam to a filter element or more particularly to the filter coating or layers. In view of this, optical filter assemblies usually comprise at least two optical fibres housed within a sleeve and one or more lenses disposed between a filter coating and the optical fibres for collimating light destined for the filter coating and for focusing light directed toward one of the optical fibres.
Optical filters such as dichroic filters are typically comprised of many layers of light transmissive high and low index material arranged to form optical cavities; the layers are extremely thin, and are in the order of the wavelength of light passing therethrough. These layers are coated or deposited on a light transmissive substrate that serves as a carrier or support for the ultra-thin coating.
Over the years, graded-index (GRIN) rod lenses have been widely used in the optics industry. They are convenient to use, and can be modified by polishing and end thereof. For example, if a pitch of a GRIN lens is too long, and requires shortening, the end face of the lens can be polished to shorten its length and effectively lessen its pitch. Due to the convenience and relatively low cost of GRIN lenses, they are most often the element of choice when manufacturing small precision optical devices. In order to couple one or more optical fibres to a GRIN lens, a fibre tube or sleeve is used having a complementary end face to and end face of the GRIN lens to which it is to be coupled. Adhesive such as epoxy is sometimes used to fixedly couple the tube with the lens in a preferred position. Heat cured epoxies are also used and tend to provide a strong immovable bond between the optical fibre sleeve and the GRIN lens. It is a well known practice, to use a similar adhesive such as UV cured adhesive, a heat cured adhesive, or metal or glass solder to fix an optical filter such as a dichroic filter to a GRIN lens or sleeve to an optical filter. However, one concern related to this practice that this invention addresses is the stress induced upon the ultra-thin coating applied to the carrier substrate. When adhesive cures, it tends to shrink and apply untoward forces upon the surface to which it is in contact with. However, for obvious reasons, it has been common practice to apply adhesive between an end face of a rod lens and filter layers to which the lens is to be coupled with.
It is an object of this invention, to lessen or obviate these problems associated with applying adhesive to the outer filter coating of a coated light transmissive substrate.
When light impinges and interacts within a multi-layer multi-cavity dichroic optical filter, the beam is often shifted within the filter layers. If this shift is not taken into account when aligning input and output optical fibres, optimum coupling will not result. One aspect of this invention addresses this problem and provides an elegant solution to optimizing coupling at the same time as obviating the requirement to apply adhesives to the coating layer of a filter.
It is therefore an object of this invention, to provide an optical filter that is less susceptible to stress and that provides suitable coupling between input and output waveguides.